Electronic scanning antenna

ABSTRACT

There is described an electronic scanning antenna comprising a multiplicity of elementary radiators arranged for example in a vertical plane and constituting an array designed for simultaneous surveillance and tracking, the array being fed by two parallel waveguides. The first waveguide is connected to the radiators through a set of first couplers in cascade with a set of fixed phase shifters. The second waveguide is connected to the radiators through a set of second couplers in cascade with a set of variable phase shifters and with the first couplers and the fixed phase shifters. The waveguides are connected to respective transmitters and receivers operating at different frequencies. The complete assembly produces simultaneously at least one tracking beam which varies in elevation and one searching beam having a fixed, preferably low elevation angle.

FIELD OF THE INVENTION

The present invention relates to an electronically scannable antenna serving to produce a plurality of directional beams which point in directions defined by different angles of elevation, these beams being capable of passing from one angle of elevation to another under the control of phase shifters associated with elementary sources or radiators transmitting outgoing electromagnetic energy or receiving part of that energy returned by a target.

BACKGROUND OF THE INVENTION

Antennas of this type, given a movement of rotation in azimuth, permit the determination of the altitude of the located objects and also the tracking of these objects. However, if it is desired to maintain a surveillance or search during tracking, it is necessary to provide a separate antenna associated with another detecting system.

In commonly owned U.S. Pat. No. 3,448,450 there has been proposed a system wherein a number of radiators or elementary sources are spaced apart vertically and excited simultaneously through respective phase shifters which produce a number of beams stacked in elevation, the angle of elevation of these beams being made variable by concurrent adjustments of the phase shifters. This system is particularly adapted for the evaluation of the altitude of tracked targets and is associated with an additional radar provided with a special antenna rotating in azimuth and effecting a surveillance.

An antenna of such system comprises a number of radiating sources spaced apart vertically and coupled through phase shifters with two feed waveguides parallel to each other. One of these waveguides is connected through a rotary coupling and a multiplexer to three transmitters operating at three different frequencies, the other of these waveguides being connected directly to the receiver of the assembly the couplers connecting that waveguide to the radiating sources impart to the incoming fields a distribution of the difference type. The assembly consequently constitutes a transmitter-receiver operating as a monopulse radar.

In order to effect a surveillance in this instance during the evaluation of the altitude of the objects or their tracking, another radar must be provided.

Thus, such an arrangement requires two different systems with distinct modes of operation, the surveillance system being a panoramic radar in which the rotation of the antenna has to be synchronized with that of the waveguides of the electronically scanned antenna.

OBJECT OF THE INVENTION

The object of the present invention is to avoid the need for an extra radar designed for surveillance and to give this function to the radar provided with the electronically scanned antenna.

SUMMARY OF THE INVENTION

I realize this object, in accordance with my present invention, by providing a first and a second waveguide paralleling a linear array of vertically stacked radiators capable of emitting and intercepting microwave energy, the two waveguides communicating with respective sets of first and second couplers at a number of spaced-apart locations corresponding to the number of radiators. A set of fixed phase shifters, respectively inserted between the first couplers and the radiators, energize the latter to emit a first beam (referred to hereinafter as a search beam) of microwave energy at an operating frequency of a first transmitter connected to the first waveguide. A set of variable phase shifters, respectively inserted between the first and second couplers in cascade with the fixed phase shifters, energize the radiators to emit a second beam (referred to hereinafter as a tracking beam) of microwave energy at an operating frequency of a second transmitter connected to the second waveguide. Whereas the search beam has a predetermined angle of elevation, the angle of the tracking beam can be selectively changed since it depends on the setting of the variable phase shifters. Intercepted echoes at the frequencies of the search beam and the tracking beam are respectively detected by a first and a second receiver connected to the first and the second waveguide.

According to another feature of my invention, a third waveguide paralleling the array of radiators may communicate with a set of third couplers connected by way of a set of invariable phase shifters to the couplers of the second waveguide so as to be in cascade with the variable phase shifters. A third receiver connected to this third waveguide detects intercepted echoes at the operating frequency of the second transmitter in a mode different from that of the second receiver, i.e., in a difference mode if the second receiver works in a summing mode.

Pursuant to a further feature of my invention, each waveguide may be provided with a plurality of transmitters of different operating frequencies connected via a multiplexer to an input of a duplexer whose output is connected via another multiplexer to a plurality of receivers. With close enough spacing of the operating frequencies of the transmitters associated with each waveguide, a plurality of fixed-elevation beams and a plurality of jointly displaceable variable-elevation beams can be generated.

BRIEF DESCRIPTION OF THE DRAWING

I shall now describe my invention in greater detail with reference to the accompanying drawing in which:

FIG. 1 is a diagram showing the fixed and movable beams obtained with an antenna according to the invention;

FIG. 2 is a block diagram of an embodiment of my invention comprising an antenna with two feed waveguides;

FIG. 3 shows the radiation patterns of the radiators fed at one and the same frequency by the feed waveguides of FIG. 2;

FIG. 4 is a graph similar to part of FIG. 3 but drawn to a larger scale and showing an angular-deviation curve;

FIG. 5 is a block diagram of another embodiment provided with an antenna having three feed waveguides;

FIGS. 6, 7 and 8 are radiation patterns of the radiators for different orders for the aiming of the movable beam;

FIG. 9 shows the curves of the distribution of the power exciting the second waveguide of FIG. 2 as a function of error; and

FIG. 10 is a block diagram of an embodiment of my invention comprising an antenna with a plurality of transmitters and receivers.

SPECIFIC DESCRIPTION

A radar system according to my invention comprises a single antenna array producing, as shown in FIG. 1, at least one fixed-elevation beam FA₁ preferably oriented at a low angle of elevation, serving for surveillance, and at least one variable elevation beam FA₂ allowing the tracking and/or the determination of the altitude of the targets which have been detected by the search beam FA₁ which is movable in azimuth.

FIG. 2 shows a block diagram of an antenna array according to the invention.

A multiplicity of elementary radiators A₁ to A_(n) for example in the form of horns, are disposed one above the other and capable of transmitting energy into space and receiving echoes from reflecting objects. These radiators are part of an assembly which rotates in azimuth about a vertical axis in the illustrated embodiment. The radiating elements may be associated with a reflector which, however, has not been shown in the Figure. These radiating elements A₁ to A_(n) are connected through phase shifters DF₁ to DF_(n), having a fixed phase-shift value, with a waveguide G₁ connected on the one hand, through a rotary coupling JT₁ and a duplexer DU₁, to a transmitter E₁ operating at a frequency f1 and, on the other hand, to a load CH₁. The connection of the several radiating elements to the waveguide is achieved by directional couplers C₁ to C_(n), respectively.

A second waveguide G₂, disposed downstream of the first waveguide G₁, is connected to the several radiators A₁ -A_(n) and the waveguide G₁ through transmission lines carrying phase shifters DV₁ to DV_(n) which are variable and controlled by an element such as a computer CT. The connection to the guide G₂ is achieved by directional couplers CD₁ to CD_(n) one branch of which is connected with a load CL₁ to CD_(n). The waveguide G₂ is connected at one of its ends, through a rotary coupling JT₂ and a duplexer DU₂, with a transmitter E₂ operating at a frequence f2 and at its other end with a load CH₂. Duplexers DU₁ and DU₂ are also connected to respective receivers R₁ and R₂.

The system shown in FIG. 2 operates in the following manner. The electromagnetic energy produced by the transmitter E₁ feeds the several radiating sources A₁ - A_(n) in series through the corresponding directional couplers C₁ - C_(n). The feed waveguide G₁ is dispersive and the seat of a progressive wave, and the direction of the maximum radiation of the resulting beam FA₁ depends on the frequency of the transmitter E₁. At this frequency f1, and with invariable phase shifts introduced by components DF₁ to DF_(n) inserted in the connections between the waveguide G₁ and the radiating elements A₁ - A_(n), it is possible to obtain a search beam FA₁ aimed in a well-determined given direction. According to the invention, this direction with a preferably low angle of elevation which is fixed in a surveillance mode.

The waveguide G₂, located downstream of the waveguide G₁, which is parallel, thereto is fed by the transmitter E₂ operating at frequency f2. With the aid of directional couplers CD₁ to CD_(n), whose main outputs are connected to respective branches of the corresponding coupling elements C₁ to C_(n) through transmission lines including respective phase shifters DV₁ - DV_(n), the elements A₁ to A_(n) radiate a second beam FA₂ independent of the first beam FA₁. In fact, the feeds of the waveguides G₁ and G₂ are independent and their inputs are decoupled. As the phase shifters DV₁ to DV_(n) respectively inserted in the transmission lines linking the waveguide G₂ with the couplers C₁ to C_(n) are variable and controlled electronically by the circuit CT, the beam FA₂ has an adjustable angle of elevation and is capable of assuming, depending on the values set in the variable phase shifters DV₁ - DV_(n), any one of a number of directions within a wide angular range substantially of the order of 50°.

It will be observed that the setting of the variable phase shifters effected at the frequency f1 will permit the beam FA₂, produced by the waveguide G₂, to point at a given instant in the same direction as the beam FA₁. This arrangement is utilized for producing in this direction, by the waveguide G₂, a difference pattern which permits the search beam FA₁ to have a reception on the sum pattern established by the waveguide G₁ and on the difference pattern established by the waveguide G₂. In this case, however, the waveguide G₂ is connected to a receiver set at the frequency f1.

In FIG. 3 I have plotted in decibels, for positive and negative angles dθ, these sum and difference patterns S₁ and D₁ respectively obtained from the waveguide G₁ for the sum and the waveguide G₂ for the difference, both waveguides operating at frequency f1.

FIG. 4 reproduces part of the curves S₁ and D₁ of FIG. 3 for a narrower angular range centered on the axis of beam FA₁ perpendicular to the array. Also shown is an angle-deviation curve T₁, plotted on a scale T, which has a large linear part in the vicinity of the axis.

Apart from this particular value for the direction assigned to the beam FA₁, the tracking beam FA₂ produced by the waveguide G₂ has a pattern of the sum type utilized of course for both transmission and reception.

FIG. 5 shows a modification of the antenna illustrated in FIG. 2 in which there has been added a third waveguide G₃ which is parallel to the first two waveguides G₁ and G₂. This waveguide G₃ is connected at one of its ends, via a rotary coupling JT₃, to a receiver R₃ set at the frequency f2 and at its other end to a load CH₃. It is connected to the radiating sources A₁ - A_(n) through couplers CP₁ to CP_(n). The connection between the couplers and the radiating sources includes a set of fixed phase shifters DP₁ to DP_(n). This connection is continued through the couplers CD₁ to C_(n), the variable phase shifters DV₁ to DV_(n), the couplers C₁ to C_(n) and the fixed phase shifters DF₁ to DF_(n).

When the radiating sources A₁ to A_(n) operate at frequency f2, they generate an overall radiation pattern of the difference type corresponding to that of the sum type produced by the transmitter E₂ and the feed waveguide G₂.

This modification consequently provides an antenna using electronic scanning which is movable in azimuth and produces in a common vertical plane a search beam and a tracking beam with a variable angle of elevation, the antenna being so arranged that, for each beam radiated in a summing mode, the reception is in the same mode at R₂ and in a difference mode at R₃.

The remaining elements of FIG. 5 correspond to those of FIG. 2.

It will be apparent that, in the modification of FIG. 5, the reception with a difference pattern produced by the waveguide G₂ at the angle of elevation of the fixed beam may be canceled.

FIGS. 6 to 9 show patterns obtained by the feeding of the antenna by the waveguide G₂ as a function of aiming orders with omission, for convenience, of the linear phase function corresponding to the aiming order. The diagrams of FIGS. 6 - 8 are drawn to similar scales allowing their superposition within the limits of a certain aiming error.

It will be observed from these figures that in the concrete case they represent, in which about 40 radiators spaced apart 83 mm in the band S are employed, this corresponds to a beam of 2° in width at half power; a beam aimed at 3° has a first lobe at 19 dB and a beam aimed at 4° has a first lobe at 23 dB.

In FIG. 6 there are shown patterns P₁, P₂ and P₃ obtained from the waveguide G₂ as a function of the aiming orders at 1°, 2° and 3°, respectively.

FIG. 7 shows patterns P₄, P₅ and P₆ for beam-aiming orders corresponding to 4°, 5° and 6° respectively. FIG. 8 shows the pattern P₇ for a beam-aiming order corresponding to 10°.

Other patterns of the same type could also be represented which would show, like those illustrated in the Figures, that the sum patterns of the radiating sources fed by the waveguide G₂ are of a quality which improves progressively as one moves away from the first beam.

FIG. 9 shows the distribution of the power between the radiators and the loads CH₁ and CH₂ at the end of the two waveguides G₁ and G₂ respectively, upon excitation of the waveguide G₂ according to the various aimining orders. From the aiming order of 3° on, the power dissipated in the load CH₁ of the waveguide G₁ is acceptable and of the order of 1 percent. In any case, with suitable design, the load CH₂ of the waveguide G₂ dissipates the same power as the load CH₁ of the waveguide G₁ when it is excited, that is to say 1 to 2% of the total power. The curve A gives the radiated power PR as a function of the aiming error of the beam FA₂ ; the curve B gives the power PD dissipated in the load CH₁ of the waveguide G.sub. 1.

The couplers linking the feed waveguides with the radiating sources are of conventional construction. They are generally constituted by waveguide junctions whose branches have coupling factors or transfer coefficients determined in such manner that the energy is correctly distributed throughout the length of the array.

Although the foregoing description has been limited to the production of one fixed-elevation beam and one variable-elevation beam, I may extend the system by multiplying the number of transmitters and receivers to obtain with the described antenna a multibeam coverage with, for example, two search beams having low angles of elevation and one or more tracking beams whose angles of elevation may be jointly varied.

FIG. 10 shows an antenna employing electronic scanning according to the invention in which it is desired to have two fixed-elevation beams and two variable-elevation beams.

This Figure repeats, with the same reference characters, a large part of FIG. 2. There has merely been added to the lower end of the waveguide G₁, beyond the rotary joint JT₁, connection from a low-level multiplexer M11 working into two receivers RM₁₁, RM₁₂ and another such connection to a high-level multiplexer M12 supplied by two transmitters EM₁₁ and EM₁₂ operating at two different frequencies.

Similarly, the duplexer DU₂ disposed at the lower end of the waveguide G₂, beyond the rotary joint JT₂, is connected to a low-level multiplexer M₂₁ and to a high-level multiplexer M₂₂. The multiplexer M₂₁ works into two receivers RM₂₁ and RM₂₂ whereas multiplexer M₂₂ is supplied by two transmitters EM₂₁ and EM₂₂ operating at two different frequencies which also differ from those of the transmitters EM₁₁ and EM₁₂ associated with the waveguide G₁.

The operation of a system such as that diagrammatically represented in FIG. 10 is not basically different from that of the system of FIG. 2 or that of FIG. 5 including a third feed waveguide G3. The energy respectively delivered by the transmitters EM₁₁ and EM₁₂ at different frequencies preferably close to each other, with suitable selection of the values of the fixed phase shifters DF₁ to DF_(n), contributes to the production of two search beams which are adjacent of each other at slightly different angles of elevation.

Likewise, the energy respectively delivered by the transmitters EM₂₁ and EM₂₂ at two different frequencies which differ from those of the transmitters EM₁₁ and EM₁₂, with an appropriate setting of the variable phase shifters DV₁ to DV_(n), contributes to the production of two beams adjacent tracking beams having jointly variable angles of elevation.

As concerns the reception, the same consideration as those discussed in conjunction with FIGS. 2 and 5 apply.

Moreover, it is evident that the number of transmitters and receivers may be different from that shown in FIG. 10, depending on the use to which the antenna according to the invention is to be put. 

What is claimed is:
 1. An electronically scanned antenna comprising:a linear array of vertically stacked radiators capable of emitting and intercepting microwave energy; a first and a second waveguide paralleling said array; a set of first couplers communicating with said first waveguide at a number of spaced-apart locations corresponding to the number of said radiators; a set of second couplers communicating with said second waveguide at a number of spaced-apart locations corresponding to the number of said radiators; first transmitter means with at least one first operating frequency connected to said first waveguide; a set of fixed phase shifters respectively inserted between said first couplers and said radiators for energizing same to emit at least one first beam of microwave energy at said first operating frequency, said first beam having a constant angle of elevation; second transmitter means with at least one second operating frequency connected to said second waveguide; a set of variable phase shifters respectively inserted between said first and second couplers in cascade with said fixed phase shifters for energizing said radiators to emit at least one second beam of microwave energy at said second operating frequency, said second beam having an angle of elevation depending on the setting of said variable phase shifters; first receiver means connected to said first waveguide for detecting intercepted echoes at said first operating frequency; and second receiver means connected to said second waveguide for detecting intercepted echoes at said second operating frequency.
 2. An antenna as defined in claim 1 wherein each of said waveguides has one end connected to the associated transmitter means and receiver means through a duplexer, the other end of each waveguide being terminated by a dissipative load.
 3. An antenna as defined in claim 2 wherein said waveguides, couplers, phase shifters and radiators form an assembly rotatable in azimuth, further comprising rotary coupling means inserted between each of said waveguides and the respective duplexer.
 4. An antenna as defined in claim 2 wherein each of said transmitter means comprises a plurality of transmitters connected via a multiplexer to an input of the respective duplexer, each of said receiver means comprising a plurality of receivers connected via another multiplexer to an output of the respective duplexer, the operating frequencies of all transmitters being different from one another, whereby a plurality of first and second beams are generated.
 5. An antenna as defined in claim 1, further comprising a third waveguide paralleling said array, a set of third couplers communicating with said third waveguide at a number of spaced-apart locations corresponding to the number of said radiators, a set of invariable phase shifters respectively inserted between said second and third couplers in cascade with said variable phase shifters, and third receiver means connected to said third waveguide for detecting intercepted echoes at said second operating frequency in a mode different from that of said second receiver means.
 6. An antenna as defined in claim 5 wherein each of said transmitter means comprises a plurality of transmitters and each of said receiver means comprises a plurality of receivers, the operating frequencies of all transmitters being different from one another. 